1. Field of the Invention
The present invention relates to a motor for an electric power steering assembly for assisting the steering force of an automotive steering wheel.
2. Description of the Related Art
FIG. 15 is a cross-section of a conventional motor for an electric power steering assembly (hereinafter xe2x80x9celectric motorxe2x80x9d) 100. The electric motor 100 comprises: a cylindrical yoke 1; two permanent field magnets 2 arranged circumferentially and secured so as to face each other inside the yoke 1; a shaft 4 disposed inside the yoke 1 by means of bearings 3 so as to be able to rotate freely; an armature 5 secured to the shaft 4; a commutator 6 comprising a plurality of copper segments 16 secured to an end portion of the shaft 4; and brushes 8 placed in contact with the surface of the commutator 6 by the elastic force of springs 7.
The armature 5 comprises: a core 9 having a plurality of slots 11 extending in the axial direction; and a winding 10 constructed by winding wiring into the slots 11 by a lap winding method.
In the above 2-pole lap-wound electric motor 100, an electric current is supplied to the winding 10 from outside by means of the brushes 8 contacting the segments 16, whereby the armature 5 rotates together with the shaft 4 due to electromagnetic action.
Since the above electric motor 100 is mainly used in relatively light-weight low-capacity automobiles, the assisting torque from the electric motor 100 is small and consequently the operating noise of the electric motor 100 is extremely smallxe2x80x94so small that it is practically unnoticeable inside the automobile.
However, now that fuel-conservation and weight reduction are required even in heavy-weight middle- and high-capacity automobiles due to public demand for fuel efficiency, reduced exhaust emissions, etc., direct-current motor power steering assemblies are starting to replace hydraulic power steering assemblies. Electric motors providing large torque are required in such cases, but since 2-pole lap-wound designs result in large-bodied motors, it is necessary to increase the number of poles to four or so to reduce size and produce high torque.
FIGS. 16 and 17 show comparisons between a 2-pole 14-slot direct-current motor (hereinafter xe2x80x9c2-pole motorxe2x80x9d) and a 4-pole 21-slot direct-current motor (hereinafter xe2x80x9c4-pole motorxe2x80x9d) given as an example of a multipolar machine. These figures show the differences in magnetic attraction acting on the armatures in 2-pole and 4-pole motors when the armatures are off center and were obtained by magnetic field analysis by the present inventors. In FIG. 16, xe2x80x9cxe2x80xa2xe2x80x9d represents the center of the stator, that is, the original center of rotation, and xe2x80x9cXxe2x80x9d represents the center of rotation when off center. In FIG. 17,  represents the force of eccentricity direction,  represents the force of right angle thereof. As can be seen from the figure, vibrations and noise are generated more easily in a 4-pole motor than in a 2-pole motor.
That is, when the forces acting on the armatures were examined with each being placed off center by the same amount (0.1 mm) from the original central position in every angle of eccentricity from 0 degrees to 360 degrees, the maximum magnetic attraction acting in the direction of eccentricity in the 4-pole motor was approximately 2.7 N, or six times the maximum magnetic attraction acting in the direction of eccentricity in the 2-pole motor which was approximately 0.45 N. In the 2-pole motor, the direction of magnetic attraction due to eccentricity can be clearly seen, and when the force acting is compared to the angle of eccentricity it is found that when the eccentricity is between the poles (an angle of eccentricity of 90 degrees or 270 degrees) approximately twice as much magnetic attraction (0.45/0.21) acts as when the eccentricity is directed towards the center of a pole (an angle of eccentricity of 0 degrees or 180 degrees). In the 4-pole motor, on the other hand, no clear direction can be seen. That is, the force in the direction of eccentricity is approximately 2.7 N for every angle of eccentricity from 0 degrees to 360 degrees, which means that there is a direction of stability with respect to eccentricity in a 2-pole motor, but no such direction exists in a 4-pole motor, and this difference can be considered to be related to the differences in vibration and noise.
Thus, it is necessary to increase the number of poles to four or so in order to reduce size and produce high torque, but problems of vibration and noise remain.
Now, apart from lap winding, wave winding may also be considered as a winding method for armatures when the number of poles is increased in order to reduce size and increase torque. With a lap winding, the number of brushes provided is generally the same as the number of poles, but with a wave winding two brushes are generally provided.
FIGS. 18 and 19 are sets of diagrams and graphs showing the magnetic attraction acting on a 4-pole 21-slot armature given as an multi-polar example, FIG. 18 showing a case with a lap winding and four brushes and FIG. 19 showing a case with a wave winding and two brushes. In FIGS. 18 and 19, xe2x80x9cxe2x97xafxe2x80x9d represents 100% current flows perpendicular to the paper in an upward direction,  represents 100% current flows perpendicular to the paper in an downward direction,  represents 50% current flows perpendicular to the paper in an downward direction,  represents 50% current flows perpendicular to the paper in an upward direction, and xe2x80x9c⊚xe2x80x9d represents current does not flow.
Comparing the two figures, we see that whereas in the case of wave winding the magnetic attraction acting on the armature as the armature turns by one slot of the core is always directed in a given radially-outward direction as indicated by the arrow A, in the case of a lap-wound 21-slot armature, the magnetic attraction moves circumferentially as indicated by the arrow B, and one problem with a lap-wound 21-slot armature is that rotational vibrations arise easily, making the generation of operating noise that much more likely.
In the case of a multi-polar odd numbered-slot lap winding, another problem is that differences arise in the electromotive forces induced among the circuits of the winding of the armature due to the influences of imbalances in the electromagnetic circuit of the yoke, eccentricities in the armature, nonuniform electric currents flowing through the brushes, engineering errors, etc., giving rise to circulating currents within the armature flowing through the brushes, and as a result the commutating action of the brushes deteriorates, leading to increases in temperature, shortened working life, increases in torque ripples in the brushes and the commutator which accompany an increase in commutation sparks generated by the brushes, as well as the combined effects thereof, thereby increasing operating noise.
At the same time, in the case of a multi-pole odd numbered-slot wave winding, there are problems such as torque ripples increasing in magnitude and workability deteriorating due to increased thickness of the winding in order to reduce the number of parallel circuits, etc.
The present invention aims to solve the above problems and an object of the present invention is to provide a motor for an electric power steering assembly enabling reduced operating noise.
In order to achieve the above object, according to one aspect of the present invention, there is provided a motor for an electric power steering assembly comprising: a yoke; a multi-polar magnetic field portion composed of at least four poles secured to the inner wall of the yoke; a shaft disposed within the yoke so as to be able to rotate freely; an armature secured to the shaft having a winding constructed by lap winding wiring into an even number of slots formed on the outer circumferential surface of a core so as to extend in the axial direction thereof; a commutator comprising a plurality of segments secured to an end portion of the shaft; and a plurality of brushes contacting the surface of the commutator.
According to one form of the present invention, there is provided a motor for an electric power steering assembly wherein the number of slots is even and is not a multiple of the number of poles.
According to another aspect of the present invention, there is provided a motor for an electric power steering assembly comprising: a yoke; a multi-polar magnetic field portion composed of at least four poles secured to the inner wall of the yoke; a shaft disposed within the yoke so as to be able to rotate freely; an armature secured to the shaft having a winding constructed by lap winding wiring into a number of slots being a multiple of the number of pairs of poles, the slots being formed on the outer circumferential surface of a core so as to extend in the axial direction thereof; a commutator comprising a plurality of segments secured to an end portion of the shaft; and a plurality of brushes contacting the surface of the commutator.
According to one form of the present invention, there is provided a motor for an electric power steering assembly wherein the number of slots is a multiple of the number of pairs of poles and is not a multiple of the number of poles.
According to another form of the present invention, there is provided a motor for an electric power steering assembly comprising equalizing members for preventing circulating currents from flowing through the brushes due to differences in induced electromotive forces arising between circuits within the circuits of the armature.
According to still another form of the present invention, there is provided a motor for an electric power steering assembly wherein Ns/(nxc3x972)xe2x89xa6Kxe2x89xa6Ns, where K is the number of equalizing members, Ns is the number of slots in the core, and n is the maximum number of segments covered by the brushes.
According to one form of the present invention, there is provided a motor for an electric power steering assembly wherein the current passing through the winding is controlled by pulse width modulation (PWM) driving.
According to another form of the present invention, there is provided a motor for an electric power steering assembly wherein the wiring is enamel-coated round wire.
According to still another form of the present invention, there is provided a motor for an electric power steering assembly wherein the magnetic field portion comprises a plurality of permanent magnets disposed so as to be spaced around the inner wall of the yoke.